It ain’t just a rock
Most aquatic insects need to return to the water as adults to lay their eggs.
Many caddisflies, such as Hydrobiosidae, are known to choose emergent rocks - rocks which stick out of the water - as their preferred place to lay eggs. Our research set out to determine whether other species of aquatic insects also use the same kind of rocks for laying eggs, and we compared streams in native forest and pasture.
We coated emergent rocks in streams and bank-side rocks (placed a few centimetres away but out of the water) with sticky Tanglefoot® to trap aquatic insects. We also coated large plastic sheets and hung them horizontally just above the stream to capture species that may drop or broadcast their eggs directly onto the water surface.
We found many adult caddisflies (both male and female) on emergent rocks in the stream (Photo 1) but almost no adult aquatic insects on bank-side rocks. We also caught many mayflies and caddisflies on the plastic sheets, including caddisfly species that lay their eggs on emergent rocks. Emergent rocks were visited by hydrobiosids and Hydropsychidae, as well as nine other caddisfly families (including Hydroptilidae) that did not strictly lay their eggs on emergent rocks. The sticky plastic sheets showed that many species of adult aquatic insects fly close to the water surface, possibly looking for suitable places to lay their eggs. We caught a total of 28 species on the sheets, compared to 16 on the rocks.
We also found that there were more emergent rocks in native forest streams than in comparable pasture streams, but egg masses (Photo 2) were more common in pasture streams. Emergent rocks in pasture streams may be in limited supply, and therefore more sought after by adult females for laying eggs. Our data suggests that emergent rocks are important places for laying eggs and also for other behaviours, which may include adult emergence (hatching), resting, courtship and mating.
Emergent rocks in streams are a potential habitat bottleneck for aquatic animals. Reducing the numbers of emergent features in pasture streams may limit suitable habitat for these aquatic species. This could potentially limit the number of egg masses overall, and increase competition for mating or resting spaces.
At the end of the day, it ain’t just a rock.